Method for determining type of sheet by imaging sheet

ABSTRACT

A first light guide guides first light in a first light guiding path. The first light is of light irradiated from the light source to a sheet. A first detection unit receives reflected light from the sheet and outputs an image signal indicating an image of a surface of the sheet. A second light guide guides second light in a second light guiding path different from the first light guiding path. The second light is of the light irradiated from the light source and is different from the first light. A second detection unit receives the second light and output a detection signal corresponding to a light amount of the second light. A control unit controls a light emission amount of the light source based on the detection signal.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of determining a type of asheet by imaging the sheet.

Description of the Related Art

There are various image forming methods such as an electrophotographicmethod and an ink jet method. Regardless of the method, imagereproducibility is improved by adjusting an image forming condition inaccordance with a sheet type (e.g., a permeability or surface property).A sheet determination device for determining a sheet type generally hasa light source for irradiating a sheet with light and a light receivingelement for receiving reflected light from the sheet. The sheet type isdetermined on the basis of an output signal of a light receiving element(Japanese Patent Laid-Open No. 2002-182518).

In order to accurately determine the type, it is necessary to correct alight amount of the illumination light irradiated onto a surface of thesheet to a target light amount. According to Japanese Patent Laid-OpenNo. 2011-013300, a light amount correction is performed based onreflected light from an inner surface reference plate disposed between alight source and a sheet.

However, in the prior art, since an inner surface reference plate isrequired, the cost of the inner surface reference plate is incurred. Inaddition, since work to mount the inner surface reference plate isnecessary, a cost associated with the increase in the number of worksteps is also necessary.

SUMMARY OF THE INVENTION

The present invention provides a sheet imaging apparatus comprising thefollowing elements. A light source. A first light guide is configured toguide first light in a first light guiding path. The first light is oflight irradiated from the light source to a sheet. A first detectionunit is configured to receive reflected light from the sheet and outputan image signal indicating an image of a surface of the sheet. A secondlight guide is configured to guide second light in a second lightguiding path different from the first light guiding path. The secondlight is of the light irradiated from the light source and is differentfrom the first light. A second detection unit is configured to receivethe second light and output a detection signal corresponding to a lightamount of the second light. A control unit is configured to control alight emission amount of the light source based on the detection signal.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for describing an image forming apparatus.

FIGS. 2A and 2B are diagrams for describing a sheet determinationdevice.

FIGS. 3A and 3B are diagrams for describing a light guide.

FIG. 4 is a diagram for describing a light guide.

FIGS. 5A to 5D are diagrams for describing output characteristics of animage capturing element.

FIGS. 6A and 6B are a flowchart illustrating an image forming method.

FIG. 7 is a diagram for describing a light guide.

FIG. 8 is a diagram for describing a light guide.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments will be described in detail with reference tothe attached drawings. Note, the following embodiments are not intendedto limit the scope of the claimed invention. Multiple features aredescribed in the embodiments, but limitation is not made an inventionthat requires all such features, and multiple such features may becombined as appropriate. Furthermore, in the attached drawings, the samereference numerals are given to the same or similar configurations, andredundant description thereof is omitted.

First Embodiment

[Image Forming Apparatus]

As shown in FIG. 1, an image forming apparatus 1 is anelectrophotographic printer. An image forming unit 50 forms a colorimage by superimposing developers (toners) of four colors of yellow (Y),magenta (M), cyan (C), and black (K). In FIG. 1, a character of Y, M, C,and K indicating a toner color is added to the end of a referencenumeral, but Y, M, C, and K characters are omitted when matters commonto the four colors are described.

A feeding cassette 2 is a storage for storing sheets P. The feedingroller 4 feeds a sheet P from the feeding cassette 2 to the conveyancepath. The conveyance roller pair 5 conveys the sheet P fed from thefeeding cassette 2 further downstream in the conveyance direction of thesheet P. A registration roller pair 6 are conveyance rollers foraligning the timing at which the sheet P arrives at the secondarytransfer roller 19 with the timing at which a toner image arrives at thesecondary transfer roller 19. A sheet sensor 22 is provided in thevicinity of the registration roller pair 6, and detects the arrivaltiming of the sheet P.

In the image forming unit 50, a photosensitive drum 11 is an imagecarrier for carrying an electrostatic latent image or a toner image. Acharging roller 12 charges the surface of the photosensitive drum 11 sothat the surface potential of the photosensitive drum 11 becomes auniform potential. An exposure device 13 forms an electrostatic latentimage by irradiating the surface of the photosensitive drum 11 withlight. A developing device 15 develops the electrostatic latent imageusing toner to form a toner image. A primary transfer unit 16 transfersthe toner image to an intermediate transfer member 17. The secondarytransfer roller 19 transfers the toner image from the intermediatetransfer member 17 to the sheet P. A fixing device 20 fixes the tonerimage on the sheet P using heat and pressure. A discharge roller 21discharges the sheet P to a tray provided outside the image formingapparatus 1.

A sheet determination device 30 determines the type of the sheet Pconveyed along the conveyance path. Here, the type is determined byspecifying a type name or obtaining a characteristic value indicating acharacteristic of the sheet P (for example: surface property, basisweight, permeability, or the like). A control unit 10 controls the motorM to adjust the conveyance speed of the sheet P. For example, in thecase of a sheet P (cardboard) having a large basis weight, theconveyance speed is made relatively slow. For sheets P of low basisweight (plain paper and thin paper) the conveyance speed is made to berelatively high. That is, the control unit 10 may control the motor Maccording to the type of the sheet P determined by the sheetdetermination device 30. In addition, the control unit 10 may controlthe temperature of the fixing device 20 according to the type of thesheet P determined by the sheet determination device 30. In addition,the control unit 10 may control the transfer voltage applied to theprimary transfer unit 16 and the secondary transfer roller 19 accordingto the type of the sheet P determined by the sheet determination device30. These are generally referred to as image forming conditions.

The type of the sheet P is not limited to a basis weight, and mayinclude a surface property. Coated paper is an example of a sheet Phaving a smooth surface. Bond paper is an example of a sheet P having acoarse surface. An electrical resistance value of the sheet P differsdepending on the surface property. A transfer condition for transferringa toner image (for example, a transfer current) is controlled inaccordance with the surface property determined by the sheetdetermination device 30. The fixing temperature for a sheet P having asmooth surface is lower than the fixing temperature for a sheet P havinga rough surface. In other words the fixing process time for a sheet Phaving a smooth surface is shorter than the fixing process time for asheet P having a rough surface. Therefore, the control unit 10 controlsimage forming conditions in accordance with the surface propertiesdetermined by the sheet determination device 30.

[Sheet Determination Device]

As shown in FIG. 2A, a sheet image capturing apparatus in the sheetdetermination device 30 includes a light source 31, a light sourcedriving unit 32, an illumination light guide 33, an image formingoptical system 34, an image capturing element 35, a monitor light guide36, and the like. The illumination light guide 33 and the monitor lightguide 36 are integrated to form an optical component 37. In the presentembodiment, the illumination light guide 33 and the monitor light guide36 are integrated, but this is merely an example. The illumination lightguide 33 and the monitor light guide 36 may be independent (separate)from each other.

When a lighting start condition is satisfied, the control unit 10instructs the light source driving unit 32 to turn on the light source31. In addition, the control unit 10 designates a light emission amountto the light source driving unit 32. The light source driving unit 32supplies a drive current (hereinafter, referred to as ILED)corresponding to the designated light emission amount) to the lightsource 31. The light source 31 is, for example, a light emitting diode.A luminous flux is emitted radially with a normal direction with respectto a light emitting surface of the light source 31 as an optical axis.Generally, there is a linear relationship between the light emissionamount of the light source 31 and the drive current ILED. The controlunit 10 adjusts the drive current ILED so that an amount of lightindicated by a detection signal outputted from the image capturingelement 35 becomes a target amount of light.

Since the light outputted from the light source 31 has a spread, a partof the light is incident on the illumination light guide 33 so as to beillumination light, and another part is incident on the monitor lightguide 36 so as to be monitor light. The illumination light guide 33 isan optical component formed of resin, and guides the illumination lightoutputted from the light source 31 to the surface of the sheet P. Themonitor light guide 36 guides the monitor light outputted from the lightsource 31 to the image capturing element 35. The image forming opticalsystem 34 forms an image of reflected light resulting from theillumination light being reflected by the surface of the sheet P on theimage capturing element 35.

The image capturing element 35 is an image sensor that receives lightthat has passed through the image forming optical system 34 and outputsan image signal. The image capturing element 35 receives the monitorlight and outputs an image signal (a detection signal) corresponding tothe light amount of the monitor light. In the present embodiment, theimage capturing element 35 is a CMOS (Complementary Metal OxideSemiconductor) sensor. That is, the image capturing element 35 has aplurality of light receiving elements (light receiving pixels) arrangedin a line. Here, the direction in which the plurality of light receivingelements are arranged and the conveyance direction of the sheet P areorthogonal to each other. The control unit 10 outputs a control signal(a trigger signal) for causing the image capturing element 35 to performimaging. Each time a trigger signal is inputted, the image capturingelement 35 performs imaging and outputs an image signal to the controlunit 10. The accumulation time of the image capturing element 35 may beset in accordance with the resolution of a surface image in theconveyance direction of the sheet P. Before imaging the sheet P, thecontrol unit 10 may acquire dark current data by turning off the lightsource 31 and causing the image capturing element 35 to perform imaging.The control unit 10 may remove noise such as disturbance light from theimage signal using the dark current data when performing thedetermination of the sheet P. In order to cancel the influence ofunevenness in the amount of light irradiated on the sheet P, unevennessin the light receiving sensitivity of the image capturing element 35,and the like, the control unit 10 may perform a shading correction. As aresult, the accuracy of determining the surface property may beimproved. In the present embodiment, such correction may be included inan imaging operation. As the image capturing element 35, a CCD (ChargeCoupled Device) sensor, a photo diode, or the like may be employed. Inthe image capturing element 35, a plurality of pixels may be arrangedover two or more columns.

The control unit 10 determines the type of the sheet P based on theimage signal outputted from the image capturing element 35. For example,the control unit 10 calculates a characteristic value of a surfaceproperty of the sheet P from variation of a plurality of receptionlevels obtained by a plurality of imaging operations. The control unit10 may determine the type of the sheet P by comparing the characteristicvalue of the surface property with a plurality of thresholds. Thecalculation of the characteristic value of the surface property may beobtained by using, for example, the maximum value or the minimum valueof the variation of the reception level, or an absolute value of thedifference thereof. Any calculation method may be employed as long as acharacteristic amount capable of determining the type of the sheet P canbe calculated. The control unit 10 decides an image forming conditionbased on the determination result. The control unit 10 controls therotation speed of a motor 3 based on the image forming condition.

The sheet sensor 22 is disposed between the registration roller pair 6and the sheet determination device 30. Based on the detection signalfrom the sheet sensor 22, the control unit 10 determines whether or notthe sheet P passes through a predetermined position in the conveyancepath. The sheet sensor 22 is an optical sensor, and detects thepresence/absence of the sheet P by shielding/passing light from thesheet P. The sheet sensor 22 may be another sheet sensor, such as anultrasonic sensor.

FIG. 2B shows elements that configure the control unit 10. A CPU 200implements various functions by executing control programs stored in thememory 212. For example, a conveyance control unit 201 controls theconveyance speed of the sheet P by controlling the rotation speed of themotor 3. An image capturing control unit 202 outputs a trigger signal tothe image capturing element 35 to thereby cause the image capturingelement 35 to perform image capturing. A light source control unit 203controls turning on/off of the light source 31, and controls the amountof light emitted from the light source 31. In particular, a correctionunit 204 corrects the light emission amount of the light source 31 inaccordance with a light amount of the monitor light detected by theimage capturing element 35. A characteristic calculation unit 205calculates a characteristic value indicating a surface property of thesheet P based on the image signal outputted from the image capturingelement 35. A determination unit 206 determines the type of the sheet Pbased on a calculation result output from the characteristic calculationunit 205. A condition deciding unit 207 decides an image formingcondition based on the calculation result output from the characteristiccalculation unit 205 or the type of the sheet P. A forming control unit208 controls the image forming unit 50 in accordance with the imageforming condition. A monitoring unit 209 monitors the position of thesheet P in the conveyance path. A counter 210 counts the drive amount ofthe motor M (for example, the number of steps). A determination unit 211determines whether or not the sheet P has arrived at the predeterminedposition based on a count value of the counter 210.

FIG. 3A is a cross-sectional view of the illumination light guide 33 ofthe sheet determination device 30. FIG. 3B is a cross-sectional view ofthe monitor light guide 36 of the sheet determination device 30. FIG. 4is a perspective view of the optical component 37. Here, depiction of ahousing (upper lid or holding portion) is omitted in order to describean arrangement in an easy to understand manner. The sheet determinationdevice 30 holds the optical component 37 and the image forming opticalsystem 34 on the housing. Note that the X-axis direction is a directionparallel to a direction in which a plurality of light receiving elementsare arranged in the image capturing element 35. The Y-axis direction isa direction parallel to the conveyance direction of the sheet P. TheZ-axis direction is a normal direction of an electric substrate 38.

As shown in FIG. 3A, the light source 31 and the image capturing element35 are mounted on the electric substrate 38. The electric substrate 38is fixed to the housing by screws. So that dust, paper powder, and thelike does not enter the housing, the housing and the optical component37 are installed without a gap therebetween. A first optical path 61 isan optical path through which the illumination light passes. The firstoptical path 61 is indicated by solid lines and dashed lines. A secondoptical path 62 is an optical path through which the monitor lightpasses. The second optical path 62 is indicated by dashed-dotted linesand solid lines. The solid lines mean being visible from each viewpoint.The dashed lines and dashed-dotted lines mean not visible from eachviewpoint. Here, only a portion of the light rays are illustrated, andluminous flux has a number of light rays passing through various opticalpaths.

As shown in FIG. 3A and FIG. 4, the illumination light guide 33 has anillumination optical system and a light receiving optical system. Theillumination optical system includes an incidence surface 51, areflection surface 52, a reflection surface 53, and a conveyance frontsurface 54. The light receiving optical system has a conveyance frontsurface 54 and a conveyance back surface 55. As shown in FIG. 3B andFIG. 4, the monitor light guide 36 has the incidence surface 51, areflection surface 52, a reflection surface 56, and the conveyance backsurface 55.

Illumination Light

As shown in FIG. 3A and FIG. 4, the illumination light, which is a partof the light emitted from the light source 31, enters the interior ofthe illumination light guide 33 from the incidence surface 51, isdeflected by the reflection surface 52, and is deflected again by thereflection surface 53, which is a curved surface. Here, the illuminationlight is collimated. Further, the illumination light is emitted from theconveyance front surface 54 to the outside of the illumination lightguide 33, and is irradiated onto a sheet P present as a surface to beirradiated. By irradiating the sheet material with the collimatedluminous flux, it is possible to more efficiently obtain thecharacteristic value of the surface property of the sheet material ascompared with the case of the divergent luminous flux. In theillumination optical system according to the embodiment, the incidentangle formed by the projected luminous flux of the illumination luminousflux (the luminous flux projected onto an XY plane parallel to the sheetP) and the conveyance direction (Y direction) of the sheet P is 45degrees. However, the incident angle can be appropriately set dependingon a type of unevenness on the surface of the sheet P, and may be, forexample, 0 degrees or 90 degrees. At this time, by increasing theincident angle of the illumination light (to more than 45 degrees),shadow due to the unevenness of the surface of the sheet P isemphasized, and the determination accuracy of the sheet P is improved.Part of the scattered light from the sheet P which is present as asurface to be irradiated enters the inside of the illumination lightguide 33 again from the conveyance front surface 54 as reflected lightoriginating as the illumination light, and is emitted to the outside ofthe illumination light guide 33 again from the conveyance back surface55. Further, the reflected light caused by the illumination light passesthrough the image forming optical system 34 and forms an image on aneffective pixel range A in the image capturing element 35.

Monitor Light

As shown in FIG. 3B and FIG. 4, the monitor light, which is another partof the light emitted from the light source 31, enters the inside of themonitor light guide 36 from the incidence surface 51 of the monitorlight guide 36. The monitor light is deflected by the reflection surface52, further deflected by the reflection surface 56, and is emitted fromthe conveyance back surface 55 to the outside of the monitor light guide36. The monitor light is incident on an effective pixel range B in theimage capturing element 35.

In the present embodiment, the effective pixel range B in the imagecapturing element 35 is employed as a light receiving element forreceiving the monitor light, but this is merely an example. Theeffective pixel range B may be realized by another light receivingelement (photoelectric conversion element) different from the imagecapturing element 35. The control unit 10 sets a maximum value of thelight amount of the monitor light irradiated onto the effective pixelrange B as the monitor light amount. For example, from a plurality oflight receiving elements configuring the effective pixel range B, lightamounts of respective light receiving elements that have beenrespectively obtained are compared with each other, and the maximumvalue is decided. The control unit 10 can detect variation in the lightamount of the light source 31 based on the monitor light amount. If thevariation of the amount of light is small, the surface of the sheet P issmooth. If the variation of the amount of light is large, the surface ofthe sheet P is coarse. Here, the maximum value of the light amount in aplurality of light receiving pixels is used as the monitor light amount,but a statistic (e.g., an average value) for a plurality of lightreceiving pixels may be used.

Here, the amount of light incident on the image capturing element 35will be described. In general, when the incident angle of theillumination light with respect to the sheet P is increased (to 45degrees or more), the light amount of scattered light from the sheet Ptoward the image capturing element 35 becomes half or less of the lightamount of the illumination light guided to the sheet P. The scatteredlight is directed to the image capturing element 35 after passingthrough the light receiving optical system provided in the illuminationlight guide 33 and the image forming optical system 34. Therefore, thelight amount of scattered light incident on the effective pixel range Ais further attenuated. Therefore, the light amount of the monitor lightincident on the effective pixel range B is also reduced to be half orless of the light amount of a light ray guided to the sheet P. That is,the reflection surface 56 is shaped so that the light amount of monitorlight incident on the effective pixel range B is substantially equal tothe light amount of illumination light incident on the effective pixelrange A. This makes it possible to detect variation in the receptionlevel of the image capturing element 35 or variation in the light amountof the monitor light with higher resolution. In the present embodiment,the reflection surface 56 is a reflection surface of a cylinder surfacehaving different optical powers depending on the reflection direction.Therefore, the light amount of monitor light incident on the effectivepixel range A is appropriately attenuated. Here, the reflection surface56 may appropriately attenuate the light amount of monitor light. Thereflection surface 56 may be, for example, an aspherically shapedreflection surface, such as an anamorphic surface, a scattering surface,or an attenuating surface. In the present embodiment, in order to detectthe light amount of the monitor light with high resolution, theattenuation amount of the reflection surface 56 is set such that anamount of light reflected by the reflection surface 56 is at least halfof or less than an amount of light incident to the reflection surface56. However, since it is sufficient if the variation of the light amountof the monitor light can be detected at a desired resolution, theattenuation amount is appropriately set depending on the requireddetection accuracy.

[Description of Light Distribution]

FIG. 5A is a graph showing an output characteristic of reception levelsobtained by performing an imaging operation using a sheet P of a certaintype. The horizontal axis represents a pixel position (a pixel address)of the image capturing element 35. The vertical axis represents thereception level. In the effective pixel range A, a part of the scatteredlight from the sheet P is incident, and each pixel in the effectivepixel range A outputs an image signal indicating a reception level.Here, the maximum value of the output value of each pixel in theeffective pixel range A is a maximum light amount A. In the effectivepixel range B, each pixel outputs an image signal indicating thereception level of the monitor light that is incident through thereflection surface 56. Here, the maximum value of the output value ofeach pixel in the effective pixel range B is a maximum light amount B.

As shown in FIG. 5A, the maximum light amount A and the maximum lightamount B are similar to each other. Further, the maximum light amount Aand the maximum light amount B are equal to or less than an allowablevalue. The allowable value is a reception level determined in advance sothat the image signal is not saturated.

[Adjustment of Light Amount of Light Source 31]

When the light amount of the light source 31 becomes excessive,reflected light from the sheet P increases. In such a case, overexposureor the like may occur on the surface of the sheet P obtained by theimage capturing element 35. Therefore, a correct characteristic value ofa surface property cannot be obtained. On the other hand, if the lightamount is insufficient, the light reflected from the sheet P becomes toosmall, and a correct surface property characteristic value cannot beobtained. As the temperature of the light source 31 rises, the lightemission amount decreases. Further, when the light source 31 becomesold, the light emission amount also decreases. Therefore, the drivecurrent ILED (in other words, the amount of light emitted by the lightsource 31), is adjusted so that the amount of light received by themonitor light becomes the target amount of light.

It is possible for the light amount of the light source 31 to changeevery time one sheet P is imaged. Therefore, a correction of the amountof light may be performed every time one sheet P is imaged. In thepresent embodiment, most portions of the optical path for guiding themonitor light are provided inside the optical component 37. That is, asecond optical path 62 is present only between the electric substrate 38and the conveyance front surface 54 of the optical component 37 in theZ-axis direction. Therefore, between the optical component 37 and thesheet P in the Z-axis direction, there is no optical path through whichthe monitor light passes. Therefore, the light amount of the monitorlight is not affected by the sheet P. That is, the detection of themonitor light and imaging of the sheet P can be performed concurrently.

The control unit 10 can detect the rate of variation of the light amountof the light source 31 due to temperature or change over time bydetecting the variation of the light amount of the monitor light.Further, detection of the monitor light which is performed by theeffective pixel range B and an imaging operation of the surface image ofa sheet P which is performed by the effective pixel range A can beperformed concurrently. Therefore, a correction of the amount of lightmay be performed every time one sheet P is imaged. Here, the correctionof the amount of light is performed every time one sheet P is imaged,but this is merely an example. The correction of the amount of light maybe performed for each of a plurality of sheets P. An execution conditionof a light amount correction can be appropriately set in accordance withthe required accuracy of the characteristic value of the surfaceproperty.

[Flowchart]

FIGS. 6A and 6B are a flowchart showing an image forming method thatincludes light amount correction. This image forming method is performedby the control unit 10.

Measurement Start Timing

In the present embodiment, a pulse motor is employed as the motor 3. Forexample, the motor 3 conveys the sheet P at a conveyance speed of 100mm/sec. The monitoring unit 209 of the control unit 10 monitors theposition of the sheet P in the conveyance path based on a detectionsignal of the sheet sensor 22 and the number of pulses (number of steps)supplied to the motor 3. The number of steps and the conveying distanceof the sheet P are in a proportional relationship. By counting thenumber of steps using the counter 210, the monitoring unit 209 predictsthe distance from the registration roller pair 6 to the leading edge ofthe sheet P based on the number of steps. The determination unit 211resets the counter to 0 when the leading edge of the sheet P is detectedby the sheet sensor 22. When the count value (the number of steps)becomes a predetermined value (e.g., 100 steps), the determination unit211 determines that the sheet P has arrived at the detection position ofthe sheet determination device 30. The predetermined value is calculatedfrom the rotation angle per step by the motor 3 and the outer diameterof the registration roller pair 6. The motor 3 is not limited to a pulsemotor. For example, a DC (direct current) motor having an encoder forcounting the number of rotations may be employed. Although a pulse motoris employed to monitor the position of the sheet P, the position of thesheet P may be monitored from other parameters. For example, when thesheet sensor 22 detects the leading edge of the sheet P, the monitoringunit 209 starts timing by a timer. The determination unit 211 may causethe sheet determination device 30 to perform a determination for thesheet P when the timer has measured a predetermined amount of time.

The control unit 10 may be configured based on the CPU 200 and controlprograms, or may be implemented by an application specific integratedcircuit (ASIC). The CPU 200 and an ASIC may execute processing in ashared manner.

Image Forming Method

In step S601, the control unit 10 (conveyance control unit 201) startsfeeding a sheet P. The control unit 10 starts driving the motor 3 tothereby cause the motor 3 to rotate the feeding roller 4, the conveyanceroller pair 5, and the registration roller pair 6. As a result,conveyance of the sheet P is started.

In step S602, the control unit 10 (light source control unit 203) setsthe value of the drive current ILED of the light source 31 to areference value Iref. The reference value Iref is set in the lightsource driving unit 32. Here, the reference value Iref is decided inadvance for example at a time of shipping the image forming apparatus 1from a factory. More specifically, the reference value Iref is decidedso that the sheet determination device 30 images the sheet P or thereference plate which serves as a reference, and the reception level inthe effective pixel range A becomes a target level. The reference valueIref is stored in a storage unit of the control unit 10. The monitorlight amount measured when the sheet P or the reference plate whichserves as a reference is imaged is referred to as a reference monitorlight amount. The reference monitor light amount is also stored in thememory 212 of the control unit 10. In the present embodiment, areference monitor light amount stored in a ROM area of the memory 212 inadvance at the time of factory shipment or the like is used, but anyvalue with which the light reception level is not saturated may be usedas the reference monitor light amount. For example, a value that was setwhen a sheet P was imaged in the past may be adopted as the referencemonitor light amount.

In step S603, the control unit 10 (light source control unit 203)outputs a lighting instruction to the light source driving unit 32, toturn on the light source 31. In step S604, the control unit 10 (theimage capturing control unit 202) outputs a trigger signal to the imagecapturing element 35 to perform an imaging operation. In step S605, thecontrol unit 10 detects the monitor light amount based on an imagesignal outputted from the effective pixel range B of the image capturingelement 35.

In step S606, the control unit 10 (light source control unit 203)determines whether the detected monitor light amount is within apredetermined range. For example, as shown in FIG. 5B, the upper limitvalue of the predetermined range may be the reference monitor lightamount×1.03, and the lower limit value of the predetermined range may bethe reference monitor light amount×0.97. That is, the predeterminedrange (target range) is a range within ±3% of the reference monitorlight amount. If the monitor light amount falls within the predeterminedrange, the control unit 10 proceeds to step S607. In contrast, if themonitor light amount does not fall within the predetermined range, thecontrol unit 10 proceeds to step S620.

In step S620, the control unit 10 (correction unit 204) corrects thedrive current ILED based on the monitor light amount. For example, thecontrol unit 10 may correct the drive current ILED by using Equation(1).

ILED (after correction)=(reference monitor light amount÷monitor lightamount×ILED (before correction)   (1)

The control unit 10 proceeds from step S620 to step S604 and performsstep S604 to step S606 again. If the monitor light amount ultimatelydoes not fall within the predetermined range, the control unit 10proceeds to step S607.

In step S607, the control unit 10 (determination unit 211) determineswhether the sheet P has arrived at a position that the sheetdetermination device 30 can detect. As described above, the control unit10 monitors the position of the sheet P using a counter or a timer. Whenthe sheet P arrives at the position that the sheet determination device30 can detect, the control unit 10 proceeds to step S608.

In step S608, the control unit 10 (the image capturing control unit 202)outputs a trigger signal to the image capturing element 35 to perform animaging operation. In step S609, the control unit 10 (the imagecapturing control unit 202) detects the monitor light amount based on animage signal outputted from the effective pixel range B of the imagecapturing element 35.

As shown in FIG. 5C, it is understood that the monitor light amount iscorrected to be within the predetermined range. Further, it isunderstood that the reflected light from the sheet P caused by theillumination light is detected in the effective pixel range A.

In step S610, the control unit 10 (characteristic calculation unit 205)calculates a characteristic value of the surface property based on theamount of light acquired by the effective pixel range A. Thiscalculation method is already known to a person skilled in the art, andtherefore a description thereof is omitted here. The determination unit206 may determine the type of the sheet P based on the characteristicvalue.

In step S611, the control unit 10 (the condition deciding unit 207)decides an image forming condition based on the characteristic value ora type of sheet P. For example, a storage unit (the memory 212) of thecontrol unit 10 may store a table for converting the characteristicvalue into an image forming condition. The control unit 10 converts thecharacteristic value into an image forming condition by referring to thetable.

In step S612, the control unit 10 (the forming control unit 208)controls the image forming unit 50 and forms an image on the sheet P. Instep S613, the control unit 10 (the forming control unit 208) determineswhether image forming for all sheets P designated by the print job hasbeen completed. When there is a next sheet P on which image formationhas not been completed, the control unit 10 sets the next sheet P to afeeding sheet, and advances the processing to step S606. On the otherhand, when the image formation on all the sheets P is completed, thecontrol unit 10 ends the series of processing according to the presentflowchart.

In the present embodiment, the light amount correction of the lightsource 31 is performed before the timing at which the leading edge ofthe sheet P passes through the registration roller pair 6. However, itsuffices if the light amount correction of the light source 31 has beencompleted prior to when the imaging operation (step S608) for obtainingthe characteristic value of the surface property of the sheets P isperformed.

According to the present embodiment, an amount of light can be obtainedwithout using an inner surface reference plate. Thus, manufacturing costshould be reduced. In addition, it is possible to perform a light amountcorrection at an arbitrary timing without depending on the conveyancestate of the sheet P. The determination accuracy of the sheet P shouldbe improved without reducing the throughput of the image formingapparatus 1.

Second Embodiment

As shown in FIG. 7, in the present embodiment, an optical component 37having two optical paths for guiding illumination light (a first opticalpath 61 and a third optical path 63) is proposed. FIG. 5D is a graphshowing an output characteristics of the reception levels. Descriptionof matters common to the first embodiment in the second embodiment willbe omitted. Since a member configuring the first optical path 61 and amember configuring the third optical path 63 are the same or similar,the reference symbol “′” may be given to a member configuring the thirdoptical path 63.

The third optical path 63 is configured by the incidence surface 51, thereflection surface 52, a reflection surface 53′, the conveyance frontsurface 54, the conveyance back surface 55, and an image forming opticalsystem 34′ of the illumination light guide 33′. Part of the lightoutputted from the light source 31 is incident on the incidence surface51, is deflected by the reflection surface 52, is further deflected bythe reflection surface 53′, and is emitted from the conveyance frontsurface 54 to the outside of the illumination light guide 33′. Here, theincident angle formed by the projection luminous flux of theillumination light and the conveyance direction with respect to the XYplane is 45 degrees. As described above, the incident angle of the firstoptical path 61 is +45 degrees. Of the illumination light, the reflectedlight reflected by the sheet P again enters the interior of theillumination light guide 33′ from the conveyance front surface 54, andis emitted from the conveyance back surface 55. The light emitted fromthe conveyance back surface 55 is incident on the effective pixel rangeC of the image capturing element 35 after passing through the imageforming optical system 34′.

Incidentally, when the sheet P is paper, there is a fiber orientationdirection (paper grain direction) in a method of manufacturing thepaper. The amount of light scattered from the sheet P changes dependingon the direction of the paper grain and the angle formed by theillumination luminous flux. That is, when the angle formed between thepaper grain direction and the conveyance direction varies, thecharacteristic value of the surface property does not stabilize, anderroneous determination of the type of the sheet P may occur. Therefore,by making the incident angle of the illumination light that has gonethrough the first optical path 61 different from the incident angle ofthe illumination light that has gone through the third optical path 63,the influence of the paper grain direction of the sheet P is reduced anderroneous determination is reduced.

In the illumination optical system according to the present embodiment,the incident angle of the illumination light that has gone through thefirst optical path 61 and the incident angle of the illumination lightthat has gone through the third optical path 63 are respectivelyplus/minus 45 degrees, but this is merely an example. It is possible toappropriately set incident angle in accordance with the type ofunevenness of the surface of the sheet P which is a determinationtarget. For example, each of these angles of incidence may be, forexample, plus/minus 30 degrees or plus/minus 60 degrees. The controlunit 10 calculates the characteristic value of the surface propertyusing the variation of the reception level of the effective pixel rangeA and the variation of the reception level of the effective pixel rangeC. As described above, the characteristic value may be calculated basedon the variation of the maximum light amount or the average light amountin the entirety of the effective pixel range A and the effective pixelrange C.

As FIG. 7 shows, because of the monitor light guide 36, the effectivepixel range B is present between the effective pixel range A and theeffective pixel range C. As shown in FIG. 5D, three peaks are generatedin the reception levels.

In the present embodiment, a case has been described in which there aretwo optical paths of the illumination light, but three or more opticalpaths may exist. Depending on the number of optical paths, the number ofreflection surfaces 53 increases.

According to the present embodiment, the effective pixel range B formonitoring the amount of light is provided between the two effectivepixel ranges A and C for detecting the characteristic value of thesurface property of the sheet P. As a result, there is a possibilitythat the optical component 37 and the image capturing element 35 can bereduced in size. As a result, miniaturization of the sheet determinationdevice 30 can be realized.

Third Embodiment

In the first embodiment and the second embodiment, an integrated opticalcomponent 37 is used. In particular, in the illumination light guide 33,it is assumed that the light emitted from the conveyance front surface54 is reflected by the sheet P and is incident on the conveyance frontsurface 54 again. However, there is a possibility that the lightscattered by any of the reflection surfaces provided in the illuminationlight guide 33 travels inside the illumination light guide 33 and isincident on the image capturing element 35. Such light may be referredto as stray light. Therefore, the third embodiment aims to reduce theinfluence of stray light by separating the optical component 37 into twooptical components.

As shown in FIG. 8, the optical component 37 is configured by an opticalcomponent 91 and an optical component 92. Note that description ofportions that are already described is omitted. The optical component91, the optical component 92, the image forming optical system 34, and alight shielding plate 71 are held by a housing (not shown). The housing,the optical component 91, the optical component 92, the image formingoptical system 34, and the light shielding plate 71 are installedwithout gaps so that dust, paper powder, or the like does not enter thehousing. The conveyance front surface 54 is separated into a conveyancefront surface 54 a provided in the optical component 91 and a conveyancefront surface 54 b provided in the optical component 92.

The optical component 91 includes the incidence surface 51, thereflection surfaces 52 and 53, and the conveyance front surface 54. Theoptical component 92 has a conveyance front surface 54 a, a conveyanceback surface 55, and a reflection surface 56.

The light shielding plate 71 is, for example, a component formed ofresin. The light shielding plate 71 is disposed so as to be sandwichedbetween the optical components 91 and 92. The light shielding plate 71removes or reduces unnecessary light (stray light) from the light source31 and the optical component 91 that goes to the image capturing element35. In the present embodiment, the light shielding plate 71 isconfigured by a component different from the housing, but the lightshielding plate 71 may be a holding member that holds the opticalcomponent 91 and the optical component 92. The holding member may be apart of the housing.

The light shielding plate 71 has a light transmission hole 58 thatallows the monitor light in the monitor light guide 36 to pass from theoptical component 91 to the optical component 92. That is, the lighttransmission hole 58 is a through hole that allows light reflected bythe reflection surface 52 and directed toward the reflection surface 56to pass through. The monitor light passing through the lighttransmission hole 58 is incident on the reflection surface 56 and isdeflected. In the present embodiment, the shape of the lighttransmission hole 58 is a quadrilateral, but this is merely an example.Any shape that allows the monitor light to pass therethrough can beemployed. The through hole may be formed from a plurality of slits. Thelight transmission hole 58 may be satisfied by a member having a lighttransmitting property. By appropriately setting the light transmittanceof this member, the light amount of monitor light incident on theeffective pixel range B may be adjusted.

Illumination Light (First Optical Path 61)

As shown in FIG. 8, the illumination light emitted from the light source31 enters the interior of the illumination light guide 33 (opticalcomponent 91) from the incidence surface 51 of the optical component 91,is deflected by the reflection surface 52, and is deflected again by thereflection surface 53. Further, the illumination light is emitted fromthe conveyance front surface 54 to the outside of the optical component91, and is irradiated onto a sheet P present as a surface to beirradiated. Part of the scattered light from the sheet P present as asurface to be irradiated again enters the interior of the illuminationlight guide 33 (optical component 92) from the conveyance front surface54 b provided on the optical component 92 as reflected light caused bythe illumination light, and is emitted to the outside of the opticalcomponent 92 from the conveyance back surface 55 again. Further, thereflected light caused by the illumination light passes through theimage forming optical system 34 and forms an image on an effective pixelrange A in the image capturing element 35.

Monitor Light (Second Optical Path 62)

As shown in FIG. 8, the monitor light emitted from the light source 31enters the inside of the optical component 91 from the incidence surface51 of the monitor light guide 36 (optical component 91). The monitorlight is deflected by the reflection surface 52, passes through thelight transmission hole 58, enters the optical component 92, and isincident on the reflection surface 56. The monitor light deflected bythe reflection surface 56 is emitted from the conveyance back surface 55of the optical component 92 to the outside of the monitor light guide 36(optical component 92). The monitor light is incident on an effectivepixel range B in the image capturing element 35.

According to the present embodiment, the optical component 37 isconfigured by the optical component 91 and the optical component 92, andthe light shielding plate 71 is disposed between the optical component91 and the optical component 92. This makes it difficult for stray lightto be incident on the effective pixel range A, the effective pixel rangeB, or the like. Therefore, it is possible to expect an improvement ofthe sheet determination accuracy of the sheet determination device 30.In addition, the degree of freedom in the arrangement of the opticalcomponent 37 in the sheet determination device 30 should be improved.

Technical Concept Derived from Embodiments

[First Aspect]

As shown in FIGS. 2 to 4, 7, and 8, and the like, the illumination lightguide 33 is an example of a first light guide that illuminates a sheetby guiding a first light (illumination light) from out of lightoutputted from a light source with respect to the sheet. That is, theillumination light guide 33 functions as a first light guide that guidesthe first light, out of the light irradiated from the light source withrespect to the sheet, through the first light guiding path. Theeffective pixel ranges A and C of the image capturing element 35function as a first detection unit (imaging unit) that receivesreflected light from the sheet and outputs an image signal indicating animage of a surface of the sheet. The effective pixel range B functionsas a detection unit that receives second light (monitor light) from outof the light outputted from the light source and outputs a detectionsignal according to the light amount of the monitor light. That is, theeffective pixel range B functions as a second detection unit thatreceives the second light and outputs a detection signal correspondingto the light amount of the second light. The monitor light guide 36functions as a second light guide for guiding the monitor light, out ofthe light outputted from the light source, to the detection unit. Thatis, the monitor light guide 36 functions as a second light guide thatguides the second light, which is different from the first light amongthe light irradiated from the light source, in a second light guidingpath that is different from the first light guiding path. The controlunit 10 functions as a control unit that controls a light emissionamount of the light source based on the detection signal. The firstoptical path 61 (first light guiding path) which is formed by the firstlight guide for guiding the illumination light from the light source tothe sheet differs from the second optical path 62 (second light guidingpath) which is formed by the second light guide for guiding the monitorlight from the light source to the detection unit. By employing such afirst light guide and a second light guide, a light amount can beobtained without using an inner surface reference plate.

[Second Aspect]

The first light guide and the second light guide may be integrated lightguide components (e.g., the optical component 37). This should reducethe number of parts and simplify the mounting work.

[Third Aspect]

The light amount of monitor light incident on the detection unit may beequal to or less than half of the light amount of illumination lightincident on the sheet. As a result, it should be possible to detect thelight amount of the monitor light with a high resolution.

[Fourth Aspect]

The first light guide may include at least one reflection surface (e.g.,reflection surfaces 52 and 53) for bending a portion of the firstoptical path. This should increase the degree of freedom in the shape ofthe first light guide.

[Fifth Aspect]

The at least one reflection surface (e.g., reflection surface 53) may bea reflection surface formed to collimate incident light. Since theillumination light becomes collimated luminous flux, the surfaceproperty of the sheet material should be efficiently detected.

[Sixth Aspect]

The second light guide may have a reflection surface (e.g., reflectionsurface 56) that functions as a scattering surface or an attenuatingsurface. This should effectively reduce the light amount of monitorlight.

[Seventh Aspect]

The reflection surface provided on the second light guide may be acurved surface. This should effectively reduce the light amount ofmonitor light.

[Eighth Aspect]

The curved surface may be an aspherical surface having different opticalpowers depending on a reflection direction. This should effectivelyreduce the light amount of monitor light.

[Ninth Aspect]

The aspherical surface may be a cylinder surface or an anamorphicsurface. This should effectively reduce the light amount of monitorlight.

[Tenth Aspect]

The reflection surface provided on the second light guide (for example,the reflection surface 56) may be a total reflection surface. Thisshould reduce the attenuation of the monitor light. In cases where thelight amount of monitor light tends to be insufficient, a totalreflection surface may be advantageous.

[Eleventh Aspect]

The reflection surface provided on the second light guide may attenuatethe monitor light and guide the monitor light to the detection unit. Asa result, it should be possible to detect the light amount of themonitor light with a high resolution.

[Twelfth Aspect]

The imaging unit and the detection unit may be the same image capturingelement. This should reduce the number of parts and decrease themounting work.

[Thirteenth Aspect]

The image capturing element may include a plurality of light receivingelements (e.g., effective pixel ranges A, B, and C) arranged in at leasta line. The first light receiving element group (e.g., effective pixelranges A and C) of the plurality of light receiving elements mayfunction as an imaging unit. The second light receiving element group(e.g., effective pixel range B) of the plurality of light receivingelements may function as a detection unit.

[Fourteenth Aspect]

A third light receiving element group (e.g., effective pixel range C) ofthe plurality of light receiving elements may function together with thefirst light receiving element group as an imaging unit. As a result, theinfluence of uneven light distribution of the optical component 37should be reduced. In addition, the influence of the paper grain of asheet P should be reduced.

[Fifteenth Aspect]

As shown in FIG. 7, the third light receiving element group may bedisposed between the first light receiving element group and the secondlight receiving element group. This should allow the size of the opticalcomponent 37 to be reduced.

[Sixteenth Aspect]

The first light guide may have the following elements. The incidencesurface 51 is an example of a first incidence surface that faces thelight source. The reflection surface 52 is an example of a firstdeflection surface that deflects the illumination light that entered thefirst light guide through the first incidence surface. The reflectionsurface 53 is an example of a second deflection surface for deflectingthe illumination light deflected by the first deflection surface. Theconveyance front surface 54 is an example of a first emission surfacewhich is disposed facing the conveyance path of the sheet and emits thelight deflected by the second deflection surface to the conveyance path.Further, the conveyance front surface 54 may be disposed so as to facethe conveyance path of the sheet, and function as a second incidencesurface onto which reflected light from the sheet is incident. Theconveyance back surface 55 is an example of a second emission surfacethat emits reflected light that entered the first light guide throughthe second incidence surface.

[Seventeenth Aspect]

The image forming optical system 34 is an example of an image formationunit for forming an image of reflected light emitted from the secondemission surface onto the imaging unit. This should enable an accurateimage showing the surface of the sheet P to be obtained.

[Eighteenth Aspect]

As shown in FIG. 8, the first light guide (e.g., illumination lightguide 33) may include a first light guide component (e.g., the opticalcomponent 91) and a second light guide component (e.g., the opticalcomponent 92). The first light guide component may have a firstincidence surface (e.g., the incidence surface 51), a first deflectionsurface (e.g., the reflection surface 52), and a second deflectionsurface (e.g., the reflection surface 53). The second light guidecomponent (e.g., the monitor light guide) may have a second incidencesurface (e.g., the conveyance front surface 54 b) and a second emissionsurface (e.g., the conveyance back surface 55).

[Nineteenth Aspect]

The light shielding plate 71 is an example of a first light shieldingmember disposed between the first light guide component and the secondlight guide component. Thus, stray light that occurs between the firstlight guide component and the second light guide component should bereduced.

[Twentieth Aspect]

The second light guide may have the following elements. The incidencesurface 51 is an example of a third incidence surface that faces thelight source. The reflection surface 52 is an example of a thirddeflection surface that deflects the monitor light that entered thesecond light guide through the third incidence surface. The reflectionsurface 56 is an example of a fourth deflection surface for deflectingthe monitor light deflected by the third deflection surface. Theconveyance back surface 55 is an example of a third emission surfacewhich is disposed parallel to the conveyance path of the sheet and emitsthe light deflected by the fourth deflection surface toward thedetection unit.

[Twenty-First Aspect]

As shown in FIG. 8, the second light guide may include a third lightguide component (e.g., the optical component 91) and a fourth lightguide component (e.g., the optical component 92). In this case, thethird light guide component may have a third incidence surface (e.g.,the incidence surface 51) and a third deflection surface (e.g., thereflection surface 52). The fourth light guide component (e.g., theoptical component 92) may have a fourth deflection surface (e.g., thereflection surface 56) and a third emission surface (e.g., theconveyance back surface 55).

[Twenty-Second Aspect]

As shown in FIG. 8, the light shielding plate 71 functions as a secondlight shielding member disposed between the third light guide componentand the fourth light guide component. The second light shielding membermay have a through hole through which the monitor light deflected by thethird deflection surface passes (e.g., the light transmission hole 58).This should reduce unnecessary light without blocking the monitor light.

[Twenty-Third Aspect]

As described in connection with step S608 or step S609, the reception ofthe reflected light from the sheet by the imaging unit and the receptionof the monitor light by the detection unit may be performedsimultaneously. This should make it possible to perform sheetdetermination and correction of the light amount of the light sourcewhile suppressing a decrease in the throughput of the image formingapparatus 1.

[Twenty-Fourth Aspect]

The sheet determination device 30 may include a sheet image capturingapparatus (e.g., the light source 31, the image capturing element 35,and the optical component 37), and a determination unit (e.g., thecontrol unit 10) that determines the type of a sheet (e.g., a typerelated to a surface property) based on the amount of light acquired bythe sheet image capturing apparatus.

[Twenty-Fifth Aspect]

The image forming apparatus 1 may include a sheet image capturingapparatus, a deciding unit (e.g., the control unit 10), and an imageforming unit (e.g., the image forming unit 50). The image formingapparatus 1 may include the sheet determination device 30, a decidingunit (e.g., the control unit 10), and an image forming unit (e.g., theimage forming unit 50). The deciding unit may determine the imageforming condition based on the image signal acquired by the sheet imagecapturing apparatus. For example, the deciding unit (e.g., the controlunit 10) may determine the image forming condition based on the typerelating to the surface property of the sheet determined by the sheetdetermination device 30. The image forming unit (for example, the imageforming unit 50) forms an image on a sheet according to an image formingcondition. As a result, an image should be appropriately formedaccording to the type of the sheet.

Various numerical values have been illustrated, but all are merelyexamples. The terms parallel and orthogonal are also not used in astrict sense. “Parallel” comprises being approximately parallel.“Orthogonal” comprises being approximately orthogonal.

The present invention is not limited to the embodiments described above,and various modifications and variations are possible without departingfrom the spirit and scope of the invention. Accordingly, the claims areappended hereto in order to make the scope of the invention public.

Other Embodiments

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully asanon-transitory computer-readable storage medium') to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc(BD)TM), a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2019-073719, filed Apr. 8, 2019 which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A sheet imaging apparatus comprising: a lightsource; a first light guide configured to guide first light in a firstlight guiding path, the first light being of light irradiated from thelight source to a sheet; a first detection unit configured to receivereflected light from the sheet and output an image signal indicating animage of a surface of the sheet; a second light guide configured toguide second light in a second light guiding path different from thefirst light guiding path, the second light being of the light irradiatedfrom the light source and different from the first light; a seconddetection unit configured to receive the second light and output adetection signal corresponding to a light amount of the second light;and a control unit configured to control a light emission amount of thelight source based on the detection signal.
 2. The sheet imagingapparatus according to claim 1, wherein the first light guide and thesecond light guide are integrated in a single light guide component. 3.The sheet imaging apparatus according to claim 1, wherein the lightamount of the second light incident on the second detection unit is lessthan or equal to half of a light amount of the first light incident onthe sheet.
 4. The sheet imaging apparatus according to claim 1, whereinthe first light guide includes at least one reflection surfaceconfigured to cause a portion of the first light guiding path to bebent.
 5. The sheet imaging apparatus according to claim 4, wherein theat least one reflection surface is formed to collimate incident light.6. The sheet imaging apparatus according to claim 1, wherein the secondlight guide includes a reflection surface that functions as a scatteringsurface or an attenuating surface.
 7. The sheet imaging apparatusaccording to claim 6, wherein the reflection surface provided on thesecond light guide is a curved surface.
 8. The sheet imaging apparatusaccording to claim 7, wherein the curved surface is an asphericalsurface having different optical powers depending on a reflectiondirection.
 9. The sheet imaging apparatus according to claim 8, whereinthe aspherical surface is a cylinder surface or an anamorphic surface.10. The sheet imaging apparatus according to claim 6, wherein thereflection surface provided on the second light guide is a totalreflection surface.
 11. The sheet imaging apparatus according to claim6, wherein the reflection surface provided on the second light guideattenuates the second light and guides the second light to the seconddetection unit.
 12. The sheet imaging apparatus according to claim 1,wherein the first detection unit and the second detection unit are thesame image capturing element.
 13. The sheet imaging apparatus accordingto claim 12, wherein the image capturing element includes a plurality oflight receiving elements arranged in at least one line, a first lightreceiving element group of the plurality of light receiving elementsfunctions as the first detection unit, and a second light receivingelement group of the plurality of light receiving elements functions asthe second detection unit.
 14. The sheet imaging apparatus according toclaim 13, wherein a third light receiving element group of the pluralityof light receiving elements and the first light receiving element groupfunction as the first detection unit.
 15. The sheet imaging apparatusaccording to claim 14, wherein the third light receiving element groupis disposed between the first light receiving element group and thesecond light receiving element group.
 16. The sheet imaging apparatusaccording to claim 1, wherein the first light guide includes a firstincidence surface that faces the light source, a first deflectionsurface configured to deflect the first light incident on the firstlight guide through the first incidence surface, a second deflectionsurface configured to deflect the first light deflected by the firstdeflection surface, a first emission surface that is disposed facing aconveyance path of the sheet and emits the light deflected by the seconddeflection surface to the conveyance path, a second incidence surface,which is disposed to face the conveyance path of the sheet and on whichreflected light from the sheet is incident, and a second emissionsurface configured to emit the reflected light incident on the firstlight guide through the second incidence surface.
 17. The sheet imagingapparatus according to claim 16, wherein an image formation unitconfigured to form an image of the reflected light emitted from thesecond emission surface on the first detection unit.
 18. The sheetimaging apparatus according to claim 17, wherein the first light guideincludes a first light guide component and a second light guidecomponent, the first light guide component includes the first incidencesurface, the first deflection surface, and the second deflectionsurface, and the second light guide component includes the secondincidence surface and the second emission surface.
 19. The sheet imagingapparatus according to claim 18, further comprising a first lightshielding member disposed between the first light guide component andthe second light guide component.
 20. The sheet imaging apparatusaccording to claim 1, wherein the second light guide includes a thirdincidence surface that faces the light source, a third deflectionsurface configured to deflect the second light incident on the secondlight guide through the third incidence surface, a fourth deflectionsurface configured to deflect the second light deflected by the thirddeflection surface, and a third emission surface configured to emit thelight deflected by the fourth deflection surface towards the seconddetection unit,
 21. The sheet imaging apparatus according to claim 20,wherein the second light guide includes a third light guide componentand a fourth light guide component, the third light guide componentincludes the third incidence surface and the third deflection surface,and the fourth light guide component includes the fourth deflectionsurface and the third emission surface.
 22. The sheet imaging apparatusaccording to claim 21, further comprising a second light shieldingmember disposed between the third light guide component and the fourthlight guide component. wherein the second light shielding member has athrough hole through which the second light deflected by the thirddeflection surface passes.
 23. The sheet imaging apparatus according toclaim 1, wherein reception of the reflected light from the sheet by thefirst detection unit, and reception of the second light by the seconddetection unit are performed at the same time.
 24. A sheet determiningapparatus comprising: a sheet imaging device that images a sheet; and adetermination unit that determines a type of the sheet based on an imagesignal obtained by the sheet imaging device, wherein the sheet imagingdevice comprises: a light source; a first light guide configured toguide, in a first light guiding path, first light of light irradiatedfrom the light source to a sheet; a first detection unit configured toreceive reflected light from the sheet and output an image signalindicating an image of a surface of the sheet; a second light guideconfigured to guide second light, which is of the light irradiated fromthe light source and is different from the first light, in a secondlight guiding path different from the first light guiding path; a seconddetection unit configured to receive the second light and output adetection signal corresponding to a light amount of the second light;and a control unit configured to control a light emission amount of thelight source based on the detection signal.
 25. An image formingapparatus comprising: a sheet imaging device that images a sheet; and adetermination unit that determines a type of the sheet based on an imagesignal obtained by the sheet imaging device, an image forming unit thatforms an image on the sheet, wherein the sheet imaging device comprises:a light source; a first light guide configured to guide, in a firstlight guiding path, first light of light irradiated from the lightsource to a sheet; a first detection unit configured to receivereflected light from the sheet and output an image signal indicating animage of a surface of the sheet; a second light guide configured toguide second light, which is of the light irradiated from the lightsource and is different from the first light, in a second light guidingpath different from the first light guiding path; a second detectionunit configured to receive the second light and output a detectionsignal corresponding to a light amount of the second light; and acontrol unit configured to control a light emission amount of the lightsource based on the detection signal.